Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

Palms and Maples

L W Doner, Eastern Regional Research Center,

Wyndmoor, PA, USA

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 The sucrose content of sap from palm trees is exten-

sively variable, but it often exceeds 10%. This is a

higher level than can be obtained from any other tree

family. Sucrose is produced in parts of tropical Africa

and Asia by evaporating water from the sap of a

variety of palm species. Sap from the North American

sugar maple contains a lower level of sucrose (1–3%).

Its natural range includes south-eastern Canada and

north-eastern USA, where maple syrup and maple

sugar are produced by evaporation of water from

the sap. (See Sucrose: Properties and Determination.)

Palm Species which Serve as Sucrose

Sources

0002 The palm family (Palmae) consists of over 2700

species. The coconut palm (Cocos nucifera) is widely

distributed, but others have a limited natural range.

Most palm species are trees, and in tropical areas they

are second only to grasses in economic importance.

Among the many food and fiber products that are

obtained is palm sugar, also known as jaggery. This

is unrefined sucrose, obtained by boiling water from,

in some cases, tree sap, and in others the sweet juice

obtained from unexpanded blossoms.

0003 A small number of the palm species account for

most of the sucrose production. The major sources

and some productivity parameters are listed in

Table 1. Some less utilized Asian sources of sucrose

include the buri palm (Corypha elata; the Philip-

pines), the nipa palm (Nipa fructicans; Malaysia),

and the gomuti palm (Arenga pinnata; Malaysia and

Java). In tropical Africa, sap from the oil palm (Elaeis

guineensis) and from palms of the genus Raphia con-

tains sucrose at about the 10% level.

Production and Utilization of Palm

Sucrose

0004Potential exists for expanding the palm sugar industry

in all growth areas. Palm sugar production remains

largely a village industry because of the difficulty in

mechanizing both the collection and the processing of

the sap. Even so, the annual production of palm

sucrose in India approaches 70 000 t, in Cambodia

35 000 t, and in Burma 20 000 t. Collection vessels

must be effectively sterilized in order to prevent

microbial contamination of the sap and conversion

of sucrose to glucose and fructose, its constituent

monosaccharides. This is often accomplished by ele-

vating the pH by the addition of lime. Collapsed sap

is most often filtered before lime addition to remove

debris, and is then evaporated over a wood fire while

being stirred in open pans.

0005Palm sap, which is produced at many Asian and

African locations, is often consumed as such or as

fermented beverages, which are sometimes distilled.

In India, for example, although raw sugar would be

the most valuable product from palm sap (neera), it is

not usually manufactured on a large scale. Instead,

most is allowed to ferment and is then consumed as a

beverage called toddy. People in India, Sri Lanka, and

other parts of South-east Asia have consumed both

toddy and distilled toddy for centuries.

0006Burma has become self-sufficient in cane sugar. In

central Burma, however, a large segment of the popu-

lation depends on palm jaggery production for its

livelihood, and about 50% of the sucrose consumed

is derived from palm. It is refined to white, crystalline

sucrose in order to compete with cane sugar. How-

ever, since unrefined palm jaggery possesses flavor

characteristics preferred by many people, a portion

should perhaps be marketed in this form. This would

tbl0001 Table 1 The major sucrose-producing palm species

Sugar palm

(Borassus flabellifer)

Date palm

(Phoenix sylvestris)

Coconut palm

(Cocos nucifera)

Sago palm

(Caryota urens)

Where cultivated South-east Asia India Throughout the tropics India, Malaysia

Tappable years 25–100 25–90 25–35 15–20

Trees per acre 500 500 80 100

Tappable months per year 4–6 4–6 6 6

Tapping frequency 2 times per day Once every 3 days 2–3 times per day 2–3 times per day

Sap yield (l per day) 23–15 2–5 2 10

Average sucrose

content of sap (%)

12 10 13 10

SUGAR/Palms and Maples 5657

be analogous to the relationship in North America

between maple sugar and cane sugar, with the former

being desired for certain specialty uses because of its

distinctive flavor.

0007 In many palm sugar production areas, forests are

being destroyed in order to provide firewood as an

energy source for evaporation of the palm sap. More

efficient means for evaporation are being tested,

including the use of solar energy.

Maple Trees as Sucrose Sources

0008 Maple sugar and syrup were high-value barter items

among Native Americans. The production area was

then, and remains, from Maine west to Minnesota

and from Quebec south to Indiana and West Virginia.

This is the natural range of the sugar maple (Acer

saccharum Marsh.), the maple species which ac-

counts for about 75% of the production. The sugar

maple and the black maple (A. nigrum Michx. F.) are

the only two of the 13 maple species native to North

America which are used to produce maple syrup.

These species are favored because their sap is much

sweeter (higher in sucrose) than the sap from the

other species.

0009 The early North American colonists found cane

sucrose to be both expensive and difficult to obtain.

As a result, they learned how to produce maple sugar.

This was increasingly relied upon during the eight-

eenth century, when coffee and tea were being con-

sumed in greater quantities. Later, when both cane

and beet sucrose became more readily available, pro-

duction of maple syrup steadily declined, especially in

the USA. At the present time Canadian production

exceeds (11 10

l) (3 10

gal) per year, nearly

double that of the USA.

Production of Maple Syrup

0010 Optimal flow of sap from maple trees occurs during

late winter and early spring, when consecutive days

with freezing temperatures at night and thawing tem-

peratures during the day are most common. Tapholes

of 95 mm diameter are drilled into the tree at a dis-

tance of 60–90 cm from the ground. The tapholes are

drilled to a depth of about 8 cm, and each tree can

accommodate up to four tapholes, depending on its

diameter.

0011 Traditionally, sap was collected by hanging

15-quart (14.19 l) buckets on the tap spout. This

continues to be a common practice among many

hobbyists, but since about 1970, commercial oper-

ations have used networks of plastic tubing to trans-

port maple sap to evaporating plants. A germicidal

pellet is often inserted into tapholes in order to inhibit

microbial growth and to insure that the tubing

remains clean and sterile. Much of the hard labor

has been eliminated by using the tubing collection

system, and production costs have been reduced by

about 40%. A typical taphole yields 5–15 gallons

(19–57 l) of sap each season. Forty gallons (151 l) of

typical sap is required to yield a gallon (3.785 l) of

maple syrup; thus an average taphole yields a quart

(0.946 l) of syrup. The syrup is produced by boiling

water from the sap until the solids (mostly sucrose)

content has increased to 67%.

Sucrose in Maple Sap and Syrup

0012Maple sap typically contains about 2% solids, of

which sucrose constitutes about 97%. The remainder

includes a variety of other organic compounds and

some inorganic salts. The composition of a typical

maple syrup is given in Table 2. Numerous other

compounds have been identified in trace amounts,

and some of these provide maple syrup with its

unique color and flavor. These compounds are gener-

ated during the sap evaporation process, which is

conducted by atmospheric boiling in open-pan evap-

orators, using fuel oil as the energy source. Maple

sugar is produced by continuing boiling until the

sucrose level exceeds 68%. Cooling this mixture

results in the rapid crystallization of maple sugar.

Maple sugar is heavily flavored and consists of su-

crose and all the other substances which were present

in the syrup.

0013Considerable research effort has been aimed at

increasing maple syrup production. Recently, this

has included the application of high-vacuum

pumping to increase sap flow, and the genetic modifi-

cation of the sugar maple with the aim of increasing

the sucrose concentration in the sap.

See also: Sucrose: Properties and Determination

tbl0002Table 2 Composition of maple syrup

Component Amount (%)

Sucrose 65–66

Water 32–33

Hexoses 0–7.9

Malic acid 0.093

Citric acid 0.010

Succinic acid 0.008

Fumaric acid 0.004

Soluble ash 0.30–0.81

Insoluble ash 0.08–0.67

Calcium 0.07

Silica 0.02

Manganese 0.005

Sodium 0.003

5658 SUGAR/Palms and Maples

Further Reading

Ali R (1960) Process development on production of sugar

from palm juice. Union of Burma Journal of Science and

Technology 2: 173–180.

Doner LW and Hicks KB (1982) Lactose and the sugars of

honey and maple. In: Lineback DR and Inglett GE (eds)

Food Carbohydrates, pp. 74–112. Westport, Connecti-

cut: AVI.

Willits CO and Hills CH (1976) Maple Syrup Producers

Manual. US Department of Agriculture, Agriculture

Handbook 134. Washington, DC: US Government

Printing Office.

Refining of Sugarbeet and

Sugarcane

M Donovan, Tate and Lyle Sugars, Reading, UK

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 Sugars, such as sucrose, glucose, and fructose, are

produced naturally in a number of fruit-bearing

plants. Sucrose is only produced commercially,

however, from sugarcane or sugarbeet. Sugarcane is

a grass that grows in tropical regions; sugarbeet is a

root crop grown in temperate climates, such as

Europe and North America. Sugar refining is the

process by which white granulated sugar is produced

from these crops. (See Fructose; Sucrose: Properties

and Determination.)

0002 The production processes are in principle the same,

but differ quite considerably in detail. The sugar from

the two sources is virtually identical. Both processes

will be described here, with separate sections where

the two processes differ.

0003 The main demand for white refined sugar has trad-

itionally been in the developed world. Cane is grown

in tropical regions, typically in Third World countries

and remote from major markets. This has resulted

in a two-stage process. The first-stage product, raw

sugar, is a part-purified, brown, crystalline material

produced in the factory. This is then transported to

the refinery, which is sited close to the consumer and

from which the product is typically pure crystalline

white sucrose, henceforth called sugar. Sugarcane is a

perishable material and must be processed almost

immediately it is cut, whereas raw sugar can be stored

and transported relatively easily.

0004 Sugarbeet is grown in temperate climates, usually

close to the consumer, and beet sugar-processing

factories are conveniently close to the farms. These

factories usually produce refined white sugar from

beet without the intermediate raw sugar stage.

0005The principal steps in both cases are the extraction

of sugar from the cane or beet by water to form an

impure sugar solution, followed by purification by a

number of techniques to remove suspended solids and

many of the dissolved impurities. The purified sugar

solution is then evaporated and crystallized to give

granulated white refined sugar. These key steps are

shown in Figure 1.

Preparation of Sugar Solution

SugarBeet Juice

0006Beet sugar processing is an operation in which sugar-

beets are the raw material and white sugar is the

primary product, and the complete process takes

place in the one factory operation.

0007Sugarbeet processing is a seasonal operation, usu-

ally operating in Europe and North America for 3 or

4 months during the period August to February; this

is the harvesting period for beet. About 7.5 tonnes of

sugarbeets are required to produce 1 tonne of refined

sugar, and the logistics and equipment to handle these

large quantities of beets are quite complex. Even a

medium-sized beet factory can process 7000 tonnes of

beet a day.

0008The sugarbeets are transferred into the plant using

a water flume system, where various devices remove

most of the foreign materials such as weeds, straw,

stones, and rocks. The final removal of foreign

material and soil is carried out in beet washers.

0009Diffusion The beets are passed through a slicer,

which produces long, thin strips called cossettes.

These are passed into a diffuser, where sugar is con-

tinuously extracted in a stream of water. A common

type of diffuser is a vertical drum. In this type the

cossettes are transported upwards by a scroll, while

the water passes down the drum, leaching sugar out

of the cossettes as it passes through them. The tem-

perature used in the diffuser is usually about 65



and the pH is maintained at about 6.5. The juice from

the diffuser is around 15% solids, and most of the

sugar in the beets is extracted. Effort is made to keep

the extraction level as high as possible, around 98%.

The cossettes after sugar extraction are called pulp,

and this is processed for animal feed.

Cane Sugar Liquor

0010The cane is shredded by knives and passes into a series

of mills where water is used to extract sugar from the

crushed cane. The sugar juice is clarified using lime to

SUGAR/Refining of Sugarbeet and Sugarcane 5659

precipitate impurities, concentrated, and then crystal-

lized. The product is termed raw sugar and is shipped

to form the raw material for the cane refinery.

0011 Raw sugar as received and processed in a cane

refinery is really quite a pure compound, although

not pure enough, and generally not prepared and

transported in sufficiently hygienic conditions to be

acceptable as a food ingredient. Raw sugar delivered

to the UK is on average 98.0% sucrose, on a dry basis.

Refined sugar, on the other hand, is 99.95% pure. At

first sight the difference between the purity of raw

and refined sugar would suggest a relatively straight-

forward process, but the impurities, including color

which in weight percentage terms is very small,

are quite difficult to remove. This is especially true

at the throughputs required for economic, modern

processing.

0012 Affination The first step in refining raw cane sugar

is affination. Raw sugar is a crystalline material, and

crystals are generally quite pure. In fact, most of the

impurities in raw sugar are in a syrup layer on the

crystal surface. The raw sugar is mixed with a hot

syrup that is at about its saturation point, in a piece of

equipment called a mingler. This is a large trough

with a horizontal helical screw, that mingles the raw

sugar and the syrup. This mixture is then centrifuged.

The most common machine is a batch basket centri-

fuge, centrifuging from 750 to 1500 kg in one batch,

each batch being processed in about 2 min. During

centrifugation a small amount of wash water is

sprayed on to the cake of sugar in the basket to

wash off some of the final layers of syrup. The

amount is kept to a minimum to prevent dissolution

of too much sugar.

0013This centrifugation produces two streams, a

washed raw sugar, and a syrup stream containing

most of the impurities from the surface of the raw

sugar. This low-purity syrup is sent to a process called

recovery. As the name implies, most of the sugar in

this stream is recovered and sent back to the main

stream for refining.

0014The washed raw sugar is about 99.6% sucrose,

showing that affination removes the bulk of the im-

purities. This sugar is then sent to the melter where it

is dissolved in water to about 65% solids, called 65



Brix.

Comparison of Beet Sugar Juice and Cane

Sugar Liquor

0015The terminology used is that a low-concentration

solution is called a juice, and a high-concentration

solution is called a liquor. At this stage in the process,

Cane

Raw sugar

Affination

Melter

Clarification

Decolorization

Crystallization

1st, 2nd, and 3rd

White refined sugar

Evaporation

Beet

Sugar beet

Diffusion

Purification/

clarification

Crystallization

1st

Recovery/

crystallization

Molasses

Crystallization

2nd and 3rd

Molasses

Evaporation

Melter

fig0001 Figure 1 Process flow diagram for refining of cane and beet sugar. Reproduced from Sugar: Refining of Sugar Beet and Sugarcane,

Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

5660 SUGAR/Refining of Sugarbeet and Sugarcane

beet sugar is in a dilute form and relatively impure. It

normally contains 85% sucrose on a dry basis, called

85% purity, and is at a concentration of about 14%

solids. The cane liquor on the other hand is at a higher

solids, usually about 64



Brix. It has a higher purity

than beet juice, that is, 99.65. This is because it has

already been crystallized once. Both solutions are

fairly highly colored; both solutions need to be sub-

jected to a purification process before they can be

crystallized to white sugar.

Purification of Beet Juice and Cane Liquor

Beet Juice Purification

0016 The juice from the diffuser contains a number of

impurities, both dissolved and as suspended solids,

and the sugar represents about 85% of the total solids

present. The purification is carried out with lime

and carbon dioxide in a series of operations which

increase the purity of the juice. The first step in some

factories is a pre-limer, where a portion of the lime is

added with control of time and temperature to pre-

cipitate much of the colloidal matter. This step is not

always used, but in all cases the balance of the lime is

added and then carbon dioxide is introduced through

the juice to precipitate calcium carbonate crystals.

Many of the impurities become absorbed on or react

with the calcium carbonate, creating precipitates,

and are thus removed. In the next step the calcium

carbonate with the impurities is sedimented and

removed by filtration. This is termed a first carbona-

tation stage. More carbon dioxide is then introduced

through the juice to remove the remainder of the lime

in a second carbonation, and the solution is filtered. A

number of different types of filter can be used, but the

filter leaf type is the most common. The calcium

carbonate ‘mud’ from the two carbonatations is

desweetened with water and filtered again on a

rotary vacuum filter. The mud is then discarded.

Cane Liquor Purification

0017 This is carried out in two stages: clarification to

remove suspended solids and some color, followed

by decolorization.

0018 Clarification Cane liquor contains the impurities

that were present in the raw sugar crystal, and the

clarification step is designed to remove the suspended

matter that is present. There are three processes than

can be used for clarification, and two of them are

also effective systems for the removal of about half

of the color in the liquor. The three different systems

are:

0019Carbonation, combined with pressure filtration

0020Phosphatation, with a clarifier using air flotation of

the scum

0021Pressure filtration using a filter aid

Carbonatation uses the addition of lime and then

bubbling of carbon dioxide through the limed sugar

liquor to precipitate calcium carbonate. In principle it

is the same as beet carbonatation, but carried out at

higher concentration and using less lime. The calcium

carbonate crystals agglomerate into a form that is an

effective filter aid and the suspended solids can be

removed on pressure filters. During the precipitation

of the calcium carbonate, the suspended solids, such

as waxes and gums, are combined into the agglomer-

ates and readily removed by filtration on pressure leaf

filters. Some of the color also reacts with the calcium,

and over 50% is removed.

0022Phosphatation also uses lime, but in this case the

precipitate is formed with phosphoric acid to make

calcium phosphate. Calcium phosphate is very diffi-

cult to filter, unlike calcium carbonate, but can be

floated off as a scum. This allows the use of a clarifier,

where air is introduced into the suction of a pump,

and the tiny air bubbles attach themselves to the flocs

of calcium phosphate, which rise to the surface. The

scum can then be removed using a scraper arrange-

ment at the top of the clarifier tank. When using a

clarifier it is not possible to obtain 100% removal of

solid, and a clarifier is often followed with a polish

filter to prevent small quantities of solid fouling the

next step, the decolorizing system.

0023Some refineries use pressure filtration with filter

aid. This is a relatively straightforward system, but

gives little color removal.

0024Decolorization Cane sugar liquor at this stage is

actually less colored than beet sugar liquor, but

because of the difference in the types of color, cane

is always passed through a decolorizing step, whereas

beet is only rarely decolorized.

0025Color can be removed from cane sugar streams by

the use of carbon, or by special resins. Often a high

degree of color removal is required, sometimes in

excess of 80%. There are several forms of carbon

decolorizer. A traditional method is to use bone

char. This is a granular material prepared by grinding

cattle bones, and roasting them in a kiln. Bone char is

90% hydroxyapatite, and 10% carbon.

0026The bone char is loaded into large vertical cylinders

called cisterns. The clarified syrup is passed down-

wards through one, and sometimes two of these cis-

terns, with the second being a polish. When the char

is exhausted, after perhaps 60 h, the flow of syrup to

the cistern is stopped. The char is washed clear of

SUGAR/Refining of Sugarbeet and Sugarcane 5661

sugar, emptied from the cistern, and sent to the drier

and then a kiln, where it is regenerated by heating to

550



C for a few minutes with a limited quantity of

air. The regenerated char is returned to a cistern and

the process is repeated.

0027 Another type of carbon decolorizer is granular

carbon. This is usually manufactured from coal by

kilning in the presence of steam. It contains 60%

carbon and it has 10 times more decolorizing capacity

than bone char. This allows the use of smaller and

generally more automated plant. A granular carbon

cistern may be run for 30 days; when it is exhausted it

is flushed with water and the carbon is kilned at about

900



C with a limited amount of oxygen.

0028 A more recent method of decolorizing sugar is by

the use of resins. These are in the form of small beads,

and the liquor is passed through cells containing these

beads. The color in sugar liquors is largely negatively

charged; thus by using an anionic resin in the chloride

form, color can be absorbed on to the pore walls of

the resin beads. The color replaces the chloride. Once

the ability of the resin to remove color has been

exhausted, it can be regenerated with common salt.

Two types of resin are commonly used, acrylic or

styrene. A resin cell can be used for up to 60 h before

needing regeneration. It is then taken offline to be

desweetened, washed, treated with salt, and put

back online.

Evaporation

0029 Both liquors need to be evaporated prior to crystal-

lization. The beet sugar juice is at 14



Brix and thus

more water needs to be removed from beet than cane

liquor.

Beet Sugar Juice Evaporation

0030 The thin juice, as it is now called, is evaporated in a

multiple-effect evaporator. This can often have as

many as five effects or stages and raises the solids

from about 14% to over 60%. A multiple effect is

used to give economic use of steam. Live steam is used

on only one of the effects, with vapor produced in

that effect boiling the liquor in another effect at re-

duced pressure, and so on. In this way the amount of

steam used is reduced by a factor of up to four. Some

of the vapor is used for other heating jobs around the

factory, such as heating the juice prior to carbonata-

tion, and boiling vacuum pans used to crystallize

sugar. The use of this vapor often dictates the tem-

peratures and pressures used in the evaporator.

0031 The product from the evaporator is about 62%

solids and 89% sucrose on a solids basis, and is called

thick juice. This is sent to a melter where crystal sugar

from second and third boilings is dissolved in it. This

is then filtered and is known as standard liquor, and is

about 74% solids and 92% purity. This is the liquor

that is sent to the vacuum pans to be crystallized to

white refined beet sugar.

Cane Sugar Liquor Evaporation

0032After decolorization, the cane sugar liquor is at 60–



Brix, and at a color low enough to be crystallized

to white granulated sugar. However, it first needs to

be evaporated at about 75



Brix for crystallization.

The traditional evaporator used in the sugar industry

has been a calandria type, i.e., a vessel with a sub-

merged bundle of tubes. More recently both falling-

film and plate-type evaporators have been used effect-

ively. Cane liquor evaporators are usually run as

double- or triple-effect, and vapor from the evapor-

ator can be used to provide the heat for the melter,

where the sugar is dissolved in water.

Crystallization

0033The evaporated sugar liquor is sent for crystalliza-

tion, although some cane liquor can be sold in this

form as liquid sugar, as it is very pure. (See Crystal-

lization: Basic Principles.)

0034Crystallization is essentially the same for beet and

cane. The purity of the syrup used to crystallize beet

sugar is lower and this, combined, with the use of

lower pressure steam, means that beet crystallizations

are slower than cane crystallizations. In the sugar

industry the term used for crystallization is boiling.

0035Sugar is crystallized in batch vessels called vacuum

pans. These can crystallize up to 70 t of sugar in one

batch, and a batch cycle can take from 2 to 4 h. In

vacuum crystallization the sugar liquor is boiled

under a vacuum until it just exceeds its saturation

point. It is then supersaturated, and crystals will

grow provided this supersaturation is maintained by

continual boiling of the liquor. In order to have some

control over the size of the crystals, the liquor is

usually seeded at a predetermined supersaturation

with a small amount of milled sugar crystals in alco-

hol. These crystals are about 10 mm in size, and 0.5–

1 l is generally added. As these crystals grow, the

deposition of sugar on them from the solution will

reduce the supersaturation. However, by heating and

evaporating the solution, water is removed and the

supersaturation is carefully controlled. If this super-

saturation becomes too high, nucleation can occur,

and the extra crystals formed will cause the size of the

final sugar to be too small. If the supersaturation is

allowed to become too low, crystal growth will slow

down or stop. Maintaining the growth of crystals at

the correct rate, and obtaining a batch of sugar at the

correct crystal size, and size dispersion has been the

5662 SUGAR/Refining of Sugarbeet and Sugarcane

skill of the pansman. Nowadays, modern measure-

ment techniques, and computer control can boil sugar

pans as well as a skilled pansman.

0036 The key features of a vacuum pan are shown in

Figure 2. The pan has a heating surface consisting of

vertical tubes or plates called a calandria, and is

heated by steam. Most pans have an agitator to assist

in circulation of the massecuite (see below). A vacuum

is maintained in the pan during the boiling to keep the

sugar in the temperature range 65–85



C. This is to

prevent color formation during the crystallization.

The yield of sugar is in the range 50–60%, and the

final mixture of crystals and concentrated sugar

liquor is called massecuite, or masse. This masse is

extremely viscous, and at the end of the crystalliza-

tion period the vacuum is released and the masse is

allowed to drop by opening a large valve at the

bottom of the vaccum pan. It is dropped into a receiv-

ing vessel, and from there it is fed to centrifuges to

separate the sugar crystals from the syrup.

0037 Continuous pans have been developed for white

refined sugar, and a few are used. However, difficul-

ties in obtaining a narrow size distribution, cost, and

some other problems mean that the vast majority of

refined sugar is boiled in batch pans.

0038 At this point there is a difference between beet and

cane. The high purity of the cane syrup at this point

means that the mother liquor from the first boiling

can be used again for another crop of white refined

sugar. In fact a total of three or four batches, includ-

ing the first batch described above, can be boiled.

With beet, only one batch can be boiled and used

directly for refined sugar. The subsequent batches,

as described below, are dissolved and put back into

the process.

0039The steam used to heat a beet vacuum pan is gener-

ated in one of the effects of the evaporator, and is

often around 100



C. In recent years a lot of effort

and capital have been spent in factories to reuse the

steam vapors generated in a number of applications

around the plant, and pans are now being boiled with

steam or vapor at quite low temperatures. Pressures

less than 1 bar abs and less than 100



C are quite

commonly used to boil beet sugar. This has required

pans to have a larger heat-transfer surface and to

have a lower hydrostatic head of liquor above the

calandria.

0040Vacuum pans for boiling cane sugar take steam

from the boiler or steam turbine backpressure at pres-

sures of 1–2 bar gage, and sometimes up to 4 bar

gage.

Centrifugation

0041White sugar is separated from mother syrup in batch

centrifuges. The massecuite is fed into a basket which

is then accelerated up to speeds in excess of 1000 rpm,

where it forms a wall of sugar that is 150–200 mm

thick. The sugar is then washed with a spray of hot

water, which dissolves a small amount of sugar from

the surface of the crystals, but assists in removing

most of the impurities such as color, ash, and invert

from the surface of the sugar crystals. The first two

need to be removed to reach purity specifications, and

removal of invert is important to prevent the sugar

from caking in storage. The centrifuge is then slowed

down, and a plough is inserted into the cake, which

discharges from the bottom of the centrifuge.

Drying

0042The moisture content of the sugar crystals from the

centrifuge is quite low, at about 1%, but this moisture

is concentrated on the surface of the crystals. It is

removed in a rotary drier, where the sugar is gently

tumbled in a stream of warm air for a residence time

of about 20 min. The most common type of sugar

drier is an inclined rotary drum, fitted with blades

or paddles to lift the sugar. The air flow can be

either cocurrent or concurrent. The final moisture is

extremely low, at 0.02%. This sugar is scalped to

remove oversized lumps, usually agglomerates of

Sight

glasses

Steam inlet

Liquor feed

Massecuite outlet

Condensate out

Vapor

out

fig0002 Figure 2 Vacuum pan for crystallizing sugar.

SUGAR/Refining of Sugarbeet and Sugarcane 5663

crystals that have built up. (See Drying: Theory of

Air-drying.)

Other Boilings

Beet Sugar

0043 In a beet factory white sugar is only produced from

the first crystallization. A second and third crystal-

lization are carried out; the crystal sugar from both of

these boilings is sent to the melter to be mixed with

thick juice (as mentioned earlier). The crystals are not

pure enough to be sold as white sugar, but are consid-

erably purer than thick juice, and thus raise the purity

of this to provide standard liquor which can be crys-

tallized to produce white sugar. A variety of names are

used for these boilings, such as raw and after product

or high and low raw.

0044 The raw boiling is carried out with the mother

liquor from the white sugar boiling, i.e., the liquor

separated from the white sugar crystals in the centri-

fuge. The afterproduct boiling is the mother liquor of

the raw boiling, although the liquor that is seeded is

usually of a higher purity. This is because it can be

difficult to get small seed to grow at low purity. The

masse from this afterproduct boiling is very viscous,

but it is still possible to crystallize more sugar from it.

The complete masse is sent to a cooling crystallizer

where it is cooled over a period of 48 h. This long

period of time extracts as much of the sugar as is

economically possible. This masse is centrifuged and

the liquor is molasses.

0045 There is still 60% sugar in beet molasses, but

owing to the solubility of sugar it is not possible to

separate any more by crystallization. New large-scale

chromatographic techniques have been developed to

separate sugar from molasses; these are not used ex-

tensively in Europe, although there are a number of

operations in the USA.

Cane Sugar Recovery

0046 The recovery process consists of a number of crystal-

lization and centrifuging steps, and is designed to

extract as much sugar as possible from low-purity

syrups. The final products are molasses and a low-

purity sugar, the latter being remelted and returned to

the main stream of the refining process, following

affination. In cane processing there are two feed

streams of low-purity syrup; the main stream is

from affination where the syrup from the surface of

the raw sugar is removed, and the other stream is

from the white crystallization process, where the

syrup that, after three or four boilings, is too impure

for extraction of white sugar is sent to recovery. The

recovery process is a series of crystallizations, usually

three, and each with a yield of 40–55%. As these

crystallizations are carried out at low purity, they

can be slow, and quite difficult. There are a number

of ways of carrying out these boilings and the schemes

can become quite complex. In one scheme the crystals

grown in the third boiling are used as the seed for the

second boiling, and the crystals from this boiling are

used as the seed for the first. However, there are

almost as many schemes as there are refineries.

0047Brown sugar can be boiled from the first, or purest

of the recovery boilings. Adjustments are made to the

purity and color of the syrup in order to obtain the

appropriate flavor and color.

0048Beet and cane molasses are sold for animal feed or

fermentation. The beet sugar industry blends a por-

tion of its molasses with the dried pulp, and makes

pellets for animal feed.

Storage

0049The drier produces a very dry sugar, with a moisture

level of about 0.02%. Nevertheless, even at this low

level there can be problems in storage, and if sugar

has to be stored for an extended period of time then it

is usually conditioned. This is carried out to prevent

the sugar caking. Conditioning is essentially a holding

period of perhaps 24 h in which any moisture released

from the crystal can pass into the surrounding air, and

be blown away from the sugar. It is usually carried

out in a silo. In some cases dehumidified air is passed

through the sugar in the silo, and in others the sugar is

passed into the silo, and out again in perhaps a 24-h

period; the movement of the sugar allows moisture

to escape without the crystals sticking together and

caking. The reason that even such a small amount of

residual moisture can cause a problem is that it is all

concentrated on the surface of the crystal. The water

is in the form of a saturated syrup, and as this syrup

crystallizes it releases moisture. If the surrounding air

is not kept moving, this moisture can humidify the air

to such an extent that any cooling can cause it to

condense. This condensed moisture can dissolve the

surface of the crystals and allow them to stick to-

gether. (See Storage Stability: Parameters Affecting

Storage Stability.)

Packaging

0050White sugar is quite free-flowing and packaging in the

standard 1-kg pack can be carried out at very high

speeds in modern equipment. This equipment takes

preprinted rolls of paper, folds and glues this paper to

form bags, fills and seals them. Brown sugars are more

difficult to package, owing to their sticky nature, and

are packaged at much lower speeds. Bulk deliveries

5664 SUGAR/Refining of Sugarbeet and Sugarcane

are typically made in tankers, 1-tonne containers and

25-kg or 50-kg bags. (See Packaging: Packaging of

Solids.)

Analysis

0051 Analysis is required of intermediate products to

ensure that the process is running correctly, and of

final products to ensure that they meet customers’

specifications.

0052 During the process a close watch is kept on concen-

tration of sugar. This is measured as Brix, which is

weight of sugar solids per weight of syrup, and in the

laboratory is measured by a refractometer. Color is

also closely monitored, measured by an absorbance of

light at 420 nm in a spectrophotometer. The ash con-

tent is monitored in cane, and in beet the purity is

followed. Purity is the amount of sucrose per unit dry

weight of substance. In both cane and beet the pH is

also measured at key points of the process. Too high

or low a pH leads to degradation of sucrose to color

byproducts or invert.

0053 The product may need several other measurements

to ensure quality and conformance to customer

requirements. The color and ash measurements are

routine requirements. For liquid sugar a Brix value

would be required. Others include a sieve analysis to

measure both the size and size distribution of the

sugar crystals, expressed as mean aperture (MA) and

coefficient of variation (CV). A test is also carried out

to measure suspended solids, which in white sugar are

extremely low.

See also: Crystallization: Basic Principles; Drying:

Theory of Air-drying; Fructose; Packaging: Packaging of

Solids; Storage Stability: Parameters Affecting Storage

Stability; Sucrose: Properties and Determination